Analysis of the RNA of avian sarcoma viruses and their nontransforming derivatives showed that all 4 major subgroups contained a and b subunits, respectively. Passage of B77 through duck embryo fibroblasts temporarily altered the ratio of a to b subunits. The group-specific polypeptides of avian myeloblastosis virus and a cloned avian sarcoma virus were identical, with the single exception of a 19,000 molecular weight polypeptide. After infection with B77 avian sarcoma virus, glucosamine blockage resulted in the appearance of a carbohydrate-deficient precursor of gp85, the major envelope protein of the virus. An avian osteopetrosis derived from avian myeloblastosis virus has been characterized with respect to its oncogenicity in vitro and its relationship to an avian lymphomatosis virus. In addition, a method for the automatic collection of P32-labeled virus from roller culture bottles has been developed. BIBLIOGRAPHIC REFERENCES: Lewandowski, L. J., R. E. Smith, D. P. Bolognesi, and M. S. Helpern. Viral protein synthesis under conditions of glucosamine block in cells transformed by avian sarcoma viruses. Virology 66:347-355. 1975 Smith, R. E., L. J. Davids, and P. E. Neiman. Comparison of an avian osteopetrosis virus with an avian lymphomatosis virus by RNA-DNA hybridization. J. Virol. 17:160-167. 1976.